[CORE] vmm_devtree: Introduce APIs for safe access to attribute values

Currently, we only had vmm_devtree_attrval() API for accessing node
attribute values as "const void *". This function returns const pointer
hence avoids prevents updates to attribute value at compile time but
this does not prevent illegal/out-of-bound reads to attribute values.

This patch introduces a wide range of APIs for safe access to node
attribute values there-by providing us a way to further improve
attribute value access from various parts of Xvisor.

Signed-off-by: Anup Patel <anup@brainfault.org>